The present invention relates generally to positioning and more particularly to a method and system for positioning with low geometrical dilution of position.
Emergency 911 service is rapidly becoming essential in today""s society. One of the compelling reasons for using the existing landline emergency 911 system is the ability to trace the caller""s location. Using databases in the telephone network switches, the caller""s location is determined and made available to the emergency services. In the event the caller is unable to inform the operator of their location, the ability to trace the call is invaluable.
The explosive growth of mobile phones, however, causes complications for emergency 911 services. While mobile users may call the 911 operator just as they would using a landline phone, there is no ability to trace the exact location of the mobile caller. The emergency 911 operator currently can only trace the mobile call to the base station closest to the mobile caller is using.
Mobile systems with the ability to locate mobile callers are known as enhanced 911 or E911 systems. One known approach to determine a mobile caller""s location involves using an improved handset. The improved handset may incorporate a global positioning systems (GPS) receiver to determine the mobile caller""s location and then transmit the location to the emergency 911 operator. Another improved handset may use signals from multiple base stations to determine the mobile caller""s location. These handset improvements, however, involve improved handset circuitry that increases the cost of the handsets. Further, the extra circuitry requires extra battery power. Moreover, deployment of the improvement takes time since it depends on the users upgrading their handsets.
Another approach would not modify the handsets, thereby avoiding the problems started above. The so-called network approach involves modifying the base stations.
The network approach may use two known positioning systems: spherical and hyperbolic. The spherical system measures the time of arrival (TOA) from the mobile signal at base stations, which are at known locations. The hyperbolic system measures the time difference of arrival (TDOA) of the mobile signal between base stations.
Positioning By TOA (Spherical System)
If each receiver knows the transmission time of the mobile and the receivers are synchronized with each other, then they can measure the travel time (or TOA) of the mobile signal, and therefore the distance from the mobile to the receiver. For example, suppose there are more than 3 receivers and the measurement by each receiver has no error. Thus, around each receiver there is a circle with each receiver as the centre and the distance between the receiver and the mobile as radius. All the circles intersect at a unique point that is the position of the mobile. The circles are known as lines of position.
Assuming the spherical system receivers are synchronized, the TOA can be estimated accurately with moderate high signal to noise ratio. The solution of the spherical equations is no very sensitive to the TOA estimation errors. Spherical systems are widely used in positioning, but they cannot be used directly to position a mobile since the mobile is not synchronized with the base stations.
Positioning by TDOA (Hyperbolic system)
TDOA can be used to position the mobile. Suppose the signal sequence arrives at base stations at (x1,y1), (x2,y2), (x3,y3) and TDOAs are measured are xcfx8421, xcfx8431, . . . , and xcexd is the speed of light, then by the wave propagation property it follows that
{square root over ((x3+L xe2x88x92x)2+L +(y3+L xe2x88x92y)2+L )}xe2x88x92{square root over ((x1+L xe2x88x92x)2+L +(y1+L xe2x88x92y)2+L )}=xcexdxcfx8431xe2x80x83xe2x80x83Eqn. 1
{square root over ((x2+L xe2x88x92x)2+L +(y2+L xe2x88x92y)2+L )}xe2x88x92{square root over ((x1+L xe2x88x92x)2+L +(y1+L xe2x88x92y)2+L )}=xcexdxcfx8421xe2x80x83xe2x80x83Eqn. 2
Therefore, Eqns. 1 and 2 define hyperbolic lines of position around the base stations if more than 3 base stations are involved in this positioning process. If there is no error for each TDOA measurement, all these hyperbolic curves will have a unique intersection, which is the position of the mobile.
Unlike the spherical system that measures TOA, the hyperbolic system measures TDOA and hence the mobile does not have to be synchronized with the base stations. Hyperbolic systems, however, are sensitive to Geometrical Dilution of Position (GDOP).
Geometrical Dilution of Position (GDOP)
There will always by the measurement errors in practice. Thus, the hyperbolas will deviate from the true curve to some extent depending on the measurement error distribution. The possible intersection of the hyperbolas might be anywhere in an area around the true location. This area is usually referred to GDOP. Thus, GDOP contributes to the overall location error. It can be shown that the GDOP is larger as the mobile location is far away from the central part of the triangular formed by base stations.
Therefore, clearly there is a need to reduce the GDOP in positioning systems.
The present invention is directed to positioning with low GDOP.
According to one aspect of the present invention, there is provided a method for positioning including the steps of: transmitting a asynchronous signal from a mobile to at least one base station; receiving the asynchronous signal at each base station; for each base station, determining a spherical line of position for the mobile using the received non-synchronous signal; and positioning the mobile using the spherical lines of position.
An advantage of the invention is less sensitivity to GDOP with a more convenient mathematical model.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.